Cathode



April 5, 1.938. A. MAYROGENIS CATHODE Filed Deo.. `11A 192e 2 sheets-sheet 1 MMoLya/ff# S v i CATHODE Filed Dec. ll, 1.926

Patentedpr. 5., 1.938l

UNITED STATES OATHODE Aristote Mavrogenis, Milwaukee, Wis., assigner to Radio Corporation of America, New York, N. Y.,

a corporation of Delaware .Application December 11, 1926, Serial No. 154,199

8 Claims.

This invention relates to cathodes for electron discharge devices and is directed to novel cathodes adapted to be used as electron emitting electrodes or elements in any electron discharge 5- device, for example in the electron discharge device of the type disclosed in my U. S. Patent No. 1,570,444 of January 19, 1926, for Wireless` apparatus; and my United States patents for Electron discharge devices, No. 1,616,914 of February 8, 1927, No. 1,629,171 of May 17, 1927, and No. 1,638,499 of August 9, 1927.

In general, this application is a continuation in part of my earlier ap-plication Serial No. 652,266, filed July 18, 1923, which has matured into United States Patent No. 1,671,598 of May 29, 1928.l

Prior to this invention, the usual practice in making equipote'ntial or indirectly heated cathodes has been to form a metal cylinder and place the heater Within the cylinder, such cylinder functioning as the cathode in the electron discharge device, or else to provide a glasstube or its equivalent through which the filament is threaded with a cylindrical metal body surrounding the tube. p

In either of these cases, difficulty is experienced due to the fact that in both cases, the active surface of the cathode is very many times greater than that of the coated or uncoated filament customarily used in vacuum tube Work. 'I'he result of this is that the inter-element capacity of the tube is so great that it precludes its elficient use for high frequency Work.

Further, Where an insulator is employed between the metal exterior and the filament, it has been found that these insulators are very fragile and are readily broken,so that it is almost an impossibility to mount and handle such a construction Without snapping the thin glass or similar tube, and further, as a slight abnormal difference of expansion of the parts is sufcient to break the tube.

Attempts have been made to utilize .a porous insulating material, such, for example, as porcelain, but the occluded gases in the porcelain can not be extracted entirely during the evacuation of the tube and it is found that when the tube is used with porcelain that gradually the vacuum is destroyed by the liberation of these occluded gases.

Further than this, when no insulator what-y soever is employed between fthe metal equipotential or indirectly heated cathode and the heater,

it is necessary to have` `zero potential difference 55 between the heater and the cathode, and also it (Cl. Z50-27.5)

is necessary to relatively Widely space the heater from the cathode. Clearly, therefore, the heater has to be brought to a higher temperature under these conditions, and is more subject to breaking and is more liable to burnout than under othe conditions.

. This invention is designed to overcome the defects noted above and objects of this invention are to provide an equipotential or indirectly heated cathode which is so constructed that'the heater, the insulator, and the cathode form in reality'a unitary non-porous structure, which is' extremely rugged and which does not appreciably occlude gases.

lA further object of this invention is to provide a ,unitary construction of equipotential or indirectly heated cathode and heater in which the cathode may be made` as small as desired, so that it can replace the ordinary filaments in vacuum tubes, if desired, and can be used in the same manner as these filaments in the various usages of vacuum tubes, such, for example, as wireless reception, rectification, or current limiting work Without unduly increasing Vthe inter-electrode capacity of the tube.

Furtherobjects are to so form the equipotential or indirectly heated cathode and heater that the danger of burning out the heater is reduced- A to a minimum as theentire body is for all prac-` tical purposes a unitary structure so that the temperature of the heater need be scarcely more than a few degreeshigher than any other portion of the unitary structure.

Y Further objects are to provide a construction for an equipotential or indirectly heated cathode, such that the evaporation of the lament is prevented and, consequently, lto lvastly prolong the effective life of theunit, and to further permit the heating of the cathode in air, if desired, in the further steps of coating or treating the cathode, as forinstance, Where dipping of the cathode into a melted bath, as in o-ne of the possible processes described later invgreater detail.

' Further objects are to provide a construction of electron-emitting cathode in which the dielectric material is rmly bound toan element of the cathode to thus reduce the size of the cathode, and materially reduce theftime required to bring the cathode up to operating temperature.

A further object is-to provide a construction of electron-emitting cathode in which the lmass of dielectric material requiredA is; very small,` whereby a lveryeiiicient transfer of` heat from the heater to the electron-emitting surface is obtained.

Further objects are to provide a construction in which a coating of dielectric material is firmly bound to one of the elements of the electronemitting cathode, and which thus produces a cathode of small size and small mass with all of the resulting advantages, and in which a dielectric coating is formed which is mechanically strong and cannot be damaged by the elements of the electron-emitting cathode.

Further objects are to provide a novel method of forming the conducting and electronically active coating upon the surface of the cathode, and forming the connections to such coating.

Embodiments of the invention are shown in the accompanying drawings in which:

Figure 1 is a View partly in section of one form of cathode, such view being very much enlarged;

Figure 2 is a detail to a-further enlarged scale of the lower portion of Figure 1;

Figure 3 is a view corresponding to Figure 1 of a modified form;

Figure 4 is a detail of the upper portion of the structure shown in Figure 3;

Figure 5-is a detail of a further modified form;

Figure 6 is a view partly in section showing the type of the tube with which the cathode is used.

Referring to Figures 1 and 2, it will be seen that the cathode comprises a lamentary heater l which is formed of tungsten,ttantalum, molybdenum, platinum, nickelor other suitable metal or. alloy thereof such materials as disclosed by 'way of example in 'my copending application Serial No; 140,848, filed October 11, 1926, and is preferably formed of a iine wire-coiled kinto a minutehelix. This heater is coated with a refractory and insulating coating, as indicated at 2, and suchcoating is provided with an enlargement 3 about centrally of each of the branches ofthe V-shaped cathode. 'I'he coating may be applied or formed by the methods disclosed in my co-pending application, Serial No. 140,848, for cathodes and method of making same filed October 11, 1926. As disclosed in my co-pending application, the dielectric material in the form of a paste or of any desired consste'ncy,is applied to the filament. The coated filament is heated or baked, and preferably a-current is passed therethrough to aid in this heating. This process is preferably carried on at a rate'to prevent bubbling or unnecessary agitation of the insulatingcoating. Further, the coating is such that it forms a strong, durable, non-cracking, non-blistering and non-hygroscopic coating over the filament. Obviously, since the dielectric material is baked'in n situ, that is, on the filament, it is hardened adwith the conductors 4, 5, and 6 embedded therein.

Also, it is to be noted that a thermionically active coating 8 is formed on the metal coating 1.

These coatings are formed in the manner hereinafter described in detail.

My co-pending application Serial No. 140,848, iiled October 11, 1926, discloses by Way of example, certain materials that may be used in' forming the insulating coating as `will appear hereinafter. v

It is to be noted that the form, shape,

length and nature of the insulating coating may be varied to meet any particular demand in connection with the form, type, shape, nature, physical condition of the outside or surface of the filament, heater or conductor, as well as in connection with the length, portion, or portions of such filament, heater, or conductor to be coated with such an insulating coating.

In forming the insulating sheath or coating 2, it is preferable to use basic compounds for the coating, such as molybdenum oxides, or hydroxids, or nickel oxides, or nickel hydroxides, or a mixture of them in which a small percentage of tungsten oxide may be added, if so desired. Further, anysuitable variety of silica, such as quartz, or a suitable silicon ycompound such as silicic oxide, feldspar, that is, a double silicate of aluminum, potassium, sodium, magnesium or calcium,

.or the like may be used. All of these materials are very finely ground, and, of course, preferably chemically pure ingredients are used.

To lthe above described base high heat resisting andinsulating materials, a small percentage of other materials may be added in order to make it more compact, elastic, mechanically strong, and non-porous, and free from cracking, blistering, or the like, and so that an air-tight strongly adherent coating may be formed. Such materials are, for instance, sodium oxide, usually used lin its compound form, tetraborate of sodium or borax, sodium nitrate, or the equivalent compounds of potassium, lead oxide, zinc oxide, double fluorite of sodium and aluminum, or calcium fluorite. Some of these materials may somewhat increase the coeicient of expansion and form powerful adherent agents Vduring heating. Other compounds may be used to vary the coeicient of expansion such, for example, as boric oxide.

It is evident, thatany other suitable compound,

or compounds, may be employed without departing from the spirit of this invention.

e It is also obvious that any other suitable form of adherent agents or binders of a different chemical natu-re from those mentioned above, may be wholly or partially substituted for the above mentioned adherent agents or binders.

It A is also Afound advantageous to add a small percent of finely ground clay. The clay can be very finely ground in water and acts as a suspension agent to keep the non-soluble material floating throughoutthe entire liquid.

The electric conductor to be used as a heater for an electron-emitting cathode, is surrounded by the selected material. Heat is applied in any suitable manner, as by passing a current through the conductor. The temperature employed is sufficient to cause the material or materials to adherently bind to the conductor, and may fuse, either partially or wholly, the material or materials employed.

'I'he heating process may be carried on in a non-oxidizing medium, such, for example, as in a vacuum, or in open air or any suitable atmosphere. However, if the. process is followed, the coating will nevertheless adherently enclose the filament, as for example by 4fusing around the filament. i j

The ends of the V-shaped heater are connected to terminals 9 which are of relatively heavy wire or otherwise made materially heavier and strongerV than the iilamentary heater.

3 .The conductors 4, 5, and 6, are very firmly embedded in the conducting coating and, consell (l quently, form a very secure and reliable electrical connection therewith. l

In the form of the invention shown in Figures 3 and 4, the same type of filamentary heater is employed, as indicated at IIJ. This heater is similarly coated with an insulating sheath I I formed with enlargements I2. Further, it is to be noted that the terminal connectors I3 in this form of the invention, have their ends embeddedy in and surrounded by the insulating sheathoorvcoating II. It is to be noted further, particularly from Figure 4, that the central part of the filamentary heater II) is connected to a terminal I4 which also is embedded in the insulating sheath and preferably such insulating-sheath is enlarged at this portion or thickened, as indicated at I I', and shown in Figure 4. A further difference from the previously described form resides in the fact that the upper portion of the insulating sheath is provided with two distinct and separate coatings formed jointly of a conducting coating I5 and a thermionically active coating I6, as previously described, but with the two portions thereof formed as separate entities and joined by the connector il. This connector also may be carried by a support or conductor I8, if desired. This cathode is adapted for use in an electron discharge device having `full wave rectification as disclosed in my United States Patent No. 1,629,171, noted above. In the form shown in Figure 5, the enlargements indicated at 3 and I2 in Figures 1 and 3, are omitted and instead, the

lconducting sheaths I9 and their coatings 20 stop short of each other and are spaced a material distance apart. The insulating sheath or coating 26 continues throughout the heater of Ysubstantially the same diameter, and is devoid of enlargements intermediate its ends.

In each form of the invention, it is to be noted that the cathode is multiple or compound in that it provides three distinct cathodes, for example, as indicated at A, B, and C in each of the figures. The cathode B, as shown in Figure k6, may be used for the cathode of the audion; the cathode C, as the cathode of the rectifier, and the cathode A, as the cathode of the current limiting device. the reference character 2l, that of the rectifier by the reference character 22, and that of the current limiting device by the reference character 23. The grid of the audion, as indicated at 24, is carried by suitable supports extending upwardly from the glass bridge piece 25 of the evacuated tube 2G, a suitable base, 21 provided with prongs being also provided as described in detail in my above noted patents. e

It is to be understood that any suitable form of supports may be used for the several portions of the apparatus. For example, as shown in Figure 6, the plates 22 and 23 may closely enclose their cathode and may have their ends turned outwardly, as indicated at 28 and 29, and rolled about and secured to the appropriate supporting rods. Similarly, the plate 2I of the audion may be formed of two similar members, and may have its ends rolled about the supports therefor, as indicated at 3U in Figure 6.

The tube is shown merely to bring out the relative position of the several `cathodes and to show the manner in which themultiple or compound cathode cooperates with andadapts itself to use in the vacuum tube in which arectifier, current limiting device and audion are employed.

It is to be distinctly understood that the leadin conductors or terminal members arepreferably The plate of the audion is indicated byv formed relatively stiff y so that they may act as supports for the cathode. e

Further, it is to be distinctly understood that although 'the terminal 'conductors have been shownas embedded, in one formof the invention,

that obviously, they may be embedded in the dielectric coatof the heater in any of the `several l forms shown. Y

The method and' apparatus for forming the insulating coating form the subject matter of my above noted copending application S. `N. 140,848.

The metallic or other electrically conducting coating is formed on the outer side of the insulating coating which may embed or enclose the o' preferablyl by heating the apparatus within a vacuum. ,Y

Other modes of coating the device to form the metal equipotential or indirectly heated cathode may comprise the use of a colloidal solution of the metals to be subsequently baked thereon. Thereafter, a third coating is formed on the metal coating and is indicated by the reference character 8 in Figure 2. This thirdcoating is formed of calcium, barium, strontium., thorium, or similar low electron aiiinity oxides or hydroxides, or theircompounds. This coating may be prepared by using a solution of ysuch materials or ltheir compoundsyand simply dipping the equipotential or indirectly heated cathodes thereinand leaving them to dry orkelse by using a: melted mass of these materials.

A further methodA of forming boththe metal equipotential or indirectly heated cathode and the coating thereon may be followed. For example, -a -solution containing a mixture of the metalliccompounds used for the main 'cathodes with oneor more of suchlowelectron aiiinity oxides maybe prepared and the insulating coating may itself bevv coated with this solution. Thereafter, the equipotential or indirectly heated cathode may be mounted in its normal` and perv cohesively binds itself, both to the oxides and to the insulating coating.

. Further, a mixture of the low electron affinity compounds and colloidal solution of the propermetals may be made and applied directly tol the insulating sheath, subsequent heating being employed in the manner described above.

It is to be noted also that there is no danger 4of burning out the heater or filament as it is coated bythe insulating strongly adherent sheath. Further, the evaporation. of the iilament is practically zero as the opening in the helix is so small that noy portionof the filament escapes during heating.` `The filament can bek heated even in the open air, if desired, although, in View ofthe fact -that the Aoxides enumerated above are lhygr-oscopic, it is preferable to heat the device in its 'permanentmounting Withinfthe -tube and While the tube is beingv exhausted. Y' 1 Other-materials `maybe used forforming the metal-coating and-oxidev coating.v 'For example, chlorides of platinum,- palladium, rhodium, ruthenium, iridium, and osmium, may -be used, or chlorides or-other compounds oflnickeL iron, cobalt, or any-mi-xture-of two or-more of these. Further, to reduce the expense of manufacture, the solution may -be mixed with metal in its powdered form, such as silver, tungsten, molybdenum, tanta-lum, iron, orv nickel, or any mixtures of these. Preferably, these metals are suspended in a colloidal form. Further, 4if desired, a small proportion of binding agent may be used vas a metal-lic lead 'oxide-or calcium chloride or any mixturethereof. -These mixtures can be applied either mixed in `a cold solution or else they may be `melted to form a melted bath into which the coated heater is dipped. The 10W affinity oxides or alkaline compounds may be added to this mixture-at the time it is formed, if desired. As a rough illustrationfof a few of the relative amounts that may -be used in groups, the-following is sub- For -example,6% oxides, 93% chlorides, and 1% binding-agent can be used satisfactorily. Another mixture may yconsistof 49% of a mixture of oxides and chlorides-in ythe relation of 10% oxides -to' 90% chlorides. To this 49%, 50% of metals,as'described above, and 1% binding agent can be added. A further -process that can be followed with these cathodes isi-to employ nickel, silver, gold, platinum,palladium, orothers of the metals mentioned above containing compoundselectrically conducting, and form-a paste-of thesecompounds using-Water, alcohol, or any liquid in Whichsuch compoundsare soluble, .or else the metals mentioned above maybe employed in'V the colloidal form and ap-plied as a colloidal Asolution to the dielectric material. `It is intended that in these cases the liquid is either -v/holly absorbed orevaporated prior to using ythe cathode.`

yAfurther method is toemploy-any of the metals just mentioned in thefform of 4-aliquid bath a in which th-e'cathode is dipped. yFurthenthe low affinity oxide coating may be formedA upon the metal coating by dipping such partially coated cathode into a melting-bath'of alkaline metals or theiroxides, or other compounds, 4particularly the metals,.such as barium, strontium, and calcium. Further, the melted solution or even the mixture may have both the-noble metal, as Well as the low electron aflinityoxides therein.

AA further method may -be-ifollowed Whichconsistsin dipping the cathode into a melted solution'containing approximately ten percentfof a1- kali metals and ninetypercent of the-main metals mentioned above. Thereaftenthe cathode, after cooling, is baked in a vacuum to the melting point of lthe oxide-s, whereby a film of oxides covers the Whole of the cathode structure and both the metallic and thermionically active coatings are formed.

Itis to be notedthat other compounds may be used in place of the low electron affinity oxides. Compounds of the alkaline metals may be used and heatedfto reduce them to low electron affinity oxides. -For example, the cathode may be dipped ina solution of-a suitable barium compound and subsequently heated to form barium (mida.

Itis to be noted that a very firm, reliable, and practical form of equipotential or'indirectly heatedca'thodeehasbcen produced by this invention, and 'that avery simple method of forming the activecoating has also been disclosed.

- It--Will beseen that vthe cathode may be used as lan'V ordinary cathode, if desired, in vacuum tube workyor may be used as thecomposite cathode described above.- In case it is used merely as a single cathode, obviously the separate portions, such as indicated at A, B, and C, in the drawings'are not formed, but either a single straight cathode is formed, orelse the entire surface of the-cathode forms one continuous member.

` It will be seen vfurther -that the cathode may be very readily handled and easily mounted in the 'tube Without danger to the parts of4 the cathode, as it is Ymaterially lsturdier than the heretofore known forms.

Further, it will be-seen thatthe cathode is not subject'to'breaking dueto changes in temperature or relativefdifferences-in temperature of the severalparts. V

It is Yobvious"thatthelamenu or heater may be of any typeor shape desired, depending upon theparticular type of tube with which it is used. Also it is clear that the characteristics of the dielectric-material will be 4varied to correspond to the particular type-of filament or heater With which `it is' associated.

It is to vbe understood that this invention is not Ylimited to any-particular form, shape, size or type of equipotential or indirectly heated cathode unit or structure.

rIfhe term coating, when used alone,` is intended to Vmean alayer of any substance covering another. i

The term adhesively is intended to mean steadilyor firmly attached.

@Although the inventionhas been described'in considerable detail, such description is intended as'illustrativel rather than limiting as the invention'may be variouslyembodied and as 'the scope of 'such invention is to be determined as claimed.

' 1. A solid Yequipotential cathode comprising a thermionicallyactive metal coating, a heat radiating element located within the metal coating, a dielectric refractory body fused as a permarient and adhesive coating on the heat radiating` element andfhermetically enclosing said heater and adhesively engaging said heater and themetallic 4`coating, and electric lead-in con-V ducto-rs'connected to the terminals of said heat radiatingelement, said lead-in conductors being relatively rigid and acting as `supports for said cathode; said lead-in conductors being enclosed in the ydielectric body portion.

2;- Amultiple, continuous, solid equipotential cathode comprising a single heating body, a refractory 'electrically insulating material fused on said` heating body and forming a linearly extending and-continuous coating on the entirelength of saidsheating body, and hermetically enclosing saidheatingbody, and a plurality of separate conductingand thermionically active coatings on said refractory insulating-material, each of said conducting coatings being electrically distinct and separate -fro'many other coating and from the heater body, vwhereby all said coatings form a permanent,'fcontinuous and adhesive whole.

3. Anelectronfdischarge devicecomprising a plurality of vcooperating electrodes, at least one ofsaidelectrodesbeingan indirectly heated electron-emitting `cathode comprising a rmetallic sleeve serving as an electron-emitter and a heater surrounded substantially by said sleeve, but maintained out of direct electrical contact therewith, said heater comprising a two-leg electric conductor anchored about its center and connected at its ends to lead-in conductors and comprising an insulating, high heat-resisting coating formed on at least said legs and at least physically adhesively bound thereto.

4. An indirectly heated electron-emitting cathode comprising a metallic sheath serving as a cathode member, a heat-emitting element substantially surrounded by said metallic sheath, an insulating, refractory coating at least partially fused on said element and thereby adhesively bound to said element and closely in contact with said metallic sheath, and an electric lead-in conductor connected to a terminal of said heat-emitting element and having its point of connection embedded within a portion of said insulating coating.

5. An indirectly heated electron-emitting cathode comprising a metallic cathode member and a heater for the same .and adjacent thereto, said heater comprising a two-leg electric conducto-r having a portion projecting from between said two legs and extending beyond said metallic cathode member and designed to be anchored to a support, said conductor being enclosed, at least at certain portions of its extent, in an insulating, high heat-resisting coating at least partially fused thereon.

6. An indirectly heated electron-emitting cathode comprising a iilament having a plurality of legs, a continuous, refractory, insulating coating at least partially fused both on at least a part of said legs and on a portion connecting one leg with the succeeding one, .an outer metallic coating formed on said insulating coating and serving as a cathode member, and an electric lead-in conducto-r having a portion embedded within said metallic coating to thus provide electrical connection therewith.

7. An electron discharge device having .an electron-emitting cathode sealed therein and comprising an electrically conductive sleeve having an outer side capable of being treated for electron-emitting purposes and serving as an electron-emitter, a heating element substantially surrounded by said conductive sleeve, but maintained out of direct electrical contact therewith, for heating said conductive sleeve, lead-in conductors, said heating element comprising two legs and being anchored at some point between its legs and being connected at its ends to said lead-in conductors, and an insulating, refractory, solid material therebetween and substantially enclosing at least a part of said legs.

8. A V-shaped heater wire, conducting leads connected to its ends, .a refractory coating fused to and surrounding each leg of said wire and the connecting portion of said leads, and a conducting sleeve surrounding each leg of the refractory coating, said sleeves being joined integrally together at the bend of the coating, and a terminal wire connected to the sleeve at said bend.

ARISTOTE MAVROGENIS. 

